The present invention generally relates to ISDN (integrated services digital network) tie line connecting systems, and more particularly to an ISDN tie line connecting system which connects the ISDN tie line via an ISDN office line.
Recently, due to the spread of services of the ISDN public network, there are demands to realize services of a primary rate interface by a private branch exchange in office lines (public network) and tie lines (private network). Such services are beginning to be used in general applications.
According to such services, it is possible to exchange more information between stations when compared to the conventional analog connections. Hence, it is possible to realize additional services such as high-speed data communication, number display of other party, and automatic calling.
FIG. 1 shows an example of a network and a primary rate interface of each apparatus in a conventional primary rate interface office line (public network). FIG. 2 shows an example of a network and a primary rate interface of each apparatus in a conventional primary rate interface tie line (private network).
In FIG. 1, a private branch exchange (PBX) 2 includes a primary rate interface (PRI) digital office line interface circuit (PRC) 4 for ISDN, a main control unit (central processing unit or CPU) 5, a main memory unit (MEM) 6 and a terminal 7. The PRC 4 of each PBX 2 is connected to an ISDN public network 3.
On the other hand, in FIG. 2, a PBX 2 includes a PRI (primary rate interface trunk or PRT) circuit 8 for tie line, a transmitter (TRNS) connected to the PRT circuit 8, a main control unit (CPU) 5, a main memory unit (MEM) 6 and a terminal 7. The TRNS 9 of the PBX 2 is connected to the TRNS 9 of the remote PBX 2 via the tie line. The TRNS 9 enables connection of remote PBXs 2.
When connecting a call, the communication is made by making the B channels and the D channel of each PRI (30B+D) correspond 1:1 between the apparatuses as indicated by (iii) and (iv) in FIG. 1 and as indicated by (viii) and (ix) in FIG. 2. The actual communication information (data) is exchanged between the apparatuses of the network in each B channel. On the other hand, the control information for controlling the connection of the communication information is exchanged between the apparatuses of the network in the D channel.
The communication information is exchanged as it is between the apparatuses of the network in each B channel. However, in the case of the PRI office line of FIG. 1, the D channel is used by converting it into the D channel of the PRI digital office line interface for the ISDN in the PRC 4 as indicated by (iii) in FIG. 1, and by converting it into the D channel of the ISDN public network in the ISDN public network 3 as indicated by (iv) in FIG. 1.
On the other hand, in the case of the PRI office line of FIG. 2, the D channel is used by converting it into the D channel of the PRI for the office line in the PRI 8 as indicated by (viii) in FIG. 2, and by converting it into the D channel of the transmitter 9 in the transmitter 9 as indicated by (ix) in FIG. 2.
Accordingly, the input and output of the PBX 2 can be made via the same PRI, but mutually different connections must be made for the case where the office line is used and for the case where the tie line is used. Hence, it is possible to effectively utilize the PRI for the case where the office line is used and for the case where the tie line is used, by making both of the connections on a common line connection.
However, the conventional ISDN tie line connecting system using the ISDN office line requires different types of connections between the PBX and the public network and between the PBXs due to the different standard restrictions, and for this reason, it is impossible to connect the public network and the private network.
Therefore, the connection of the PBX to the ISDN public network and the connection of the PBX to the private network must be made independently. As a result, although various useful functions are realized, the system is not efficient and the cost of the service is high.
On the other hand, the PRI provides a plurality of channels, such as 23 or 30 channels, in one interface. Hence, if the PRI is used with a PBX having a relatively small capacity, there is no need to use all of the channels provided, and as a result, a relatively large number of unused channels exist within one PRI. In other words, if the ISDN public network connection using the PRI and the private network connection using the PRI are used simultaneously in the PBXs having the relatively small capacity, there is a problem in that a large part of the PRIs are unused and wasted.
The following two methods are conceivable as methods of solving the above described problem.
According to a first method (a), the information of the digital tie line is exchanged in the D channel which is used in the ISDN public network.
According to a second method (b), the information of the digital tie line is exchanged in the B channels used in the ISDN public network.
However, the service provided by the first method (a) is dependent on the information transmission capability of the D channel of the ISDN public network. For this reason, the quality of the service deteriorates compared to the case where the direct connection is employed.
On the other hand, the following methods are conceivable as the second method (b). That is, it is conceivable (b1) to control the generation and exchange of the D channel information by the main control program of the PBX or, (b2) to loop back the digital tie line interface to the switch network of the system so as to connect to the ISDN network.
However, the method (b1) must exchange the D channel data by a high-speed data communication which cannot be fully supported by the main control program of the PBX, and the system structure of the existing PBX itself must be modified. The method (b2), on the other hand, only requires a loop-back connection circuit on an external interface circuit of the PBX and a fixed connection within the PBX. Accordingly, this method (b2) is more promising compared to the methods (a) and (b1), and it would be convenient if such a method (b2) could be realized.